1. Field of the Invention
The present invention relates to thin film electronic devices and, more specifically, to a system for making thin film graphitic devices.
2. Description of the Prior Art
In modern microelectronics integrated-circuit technology, a silicon wafer is lithographically patterned to accommodate a large number of interconnected electronic components (field effect transistors, resistors, capacitors, etc). The technology relies on the semiconducting properties of silicon and on lithographic patterning methods. Increasing the density of electronic components and reducing the power consumption per component are two of the most important objectives in the microelectronics industry, which has driven the steady reduction in the size of the components in the past decades. However, miniaturization of silicon-based electronics will reach an ultimate limit in the near future, primarily because of limitations imposed by the material properties of silicon, and doped silicon, at the nanoscale.
To sustain the current trend in microelectronics beyond the limits imposed by silicon-based microelectronics technologies, alternative technologies need to be developed. Requirements for such an alternative technology include: smaller feature sizes than feasible with silicon-based microelectronics, more energy-efficient electronics strategies, and production processes that allow large-scale integration, preferably using lithographic patterning methods related to those used in silicon-based microelectronics fabrication.
Several alternatives to silicon-based electronics have been proposed. However, none of the proposed alternatives fulfills all three of the above-listed requirements. For example, molecular electronics is considered to be an attractive alternative to silicon-based electronics. Molecular electronics devices will rely on electronic transport properties through molecules.
One proposed example of molecular electronics employs carbon nanotubes, which are considered to be particularly attractive candidates as building blocks of molecular electronics. Carbon nanotubes are essentially graphite tubes consisting of one to about 100 graphene layers in tubular configurations. A graphene layer consists of a single layer of carbon atoms arranged in a hexagonal pattern where each atom (except those at the edges) is chemically connected to its three neighbors by sp2 bonds. Crystalline graphite consists of stacked graphene layers.
The electronic transport properties of carbon nanotubes are due to the π bands of the graphene network. Hence, the electronic properties are directly related to their graphitic structure. Properties of nanotubes include the following: they conduct electrons in either a metallic mode or a semiconducting mode depending on their specific structure; they have been found to be one-dimensional ballistic conductors over micron-scale distances at room temperature; the bandgap of semiconducting nanotubes depends on the diameter of the nanotube, hence it can be tuned depending on its width; they can sustain very large currents (up to 1 mA); they are essentially graphitic and the sp2 graphite bond ranks among the strongest in nature, making nanotubes exceptionally stable compared to other molecules; and they have been shown to be capable of forming field-effect transistors. Small integrated circuits, involving up to three carbon nanotubes have been demonstrated. These structures consist of several carbon nanotubes that are deposited on an insulating substrate and interconnected with metal wires that are lithographically patterned on top of the nanotubes.
Despite the advantages mentioned above, there are also important disadvantages associated with carbon nanotube-based molecular electronics. For example, since nanotubes are either metallic or semiconducting they must be pre-selected before they are positioned on the substrate. This aspect by itself currently prohibits large-scale integration of nanotubes. Also, present nanotube configurations are interconnected with metal wires. The Ohmic resistance at each metal-to-nanotube contact is quite large. For example, in the “on” condition, each carbon nanotube transistor exhibits a resistance of several kilo Ohms which means that relatively large amounts of heat are dissipated at the contacts compared with silicon transistors.
Because of these disadvantages, nanotubes are not used yet in commercial integrated electronic circuits. Moreover, integration of carbon nanotube-based electronic devices on a large scale is not expected to be feasible in the foreseeable future.
Nano-patterned epitaxial graphene electronics (NPEG electronics) requires that single- or multi-layered graphene or ultra thin graphite is patterned in order to produce electronically active structures. Ultra thin graphitic layers include graphitic structures from a single graphene layer to up to 100 graphene layer. Ultra thin graphene includes both multilayered graphene as well as multilayered graphite. The generic patterned multilayered graphene (also known as ultra thin graphite) structure consists of one or more graphitic leads that connect to π conjugated structures. These π conjugated electronic systems may be formed ‘ by patterning the epitaxial graphene multi-layers directly using patterning methods (known as the top-down approach).
It is desirable that electronic structures can be patterned at the nanoscale. However, very small structures become increasingly sensitive to variations as structures that are different by the position of only a few atoms may significantly alter the electronic performance of the structure. Consequently, to insure that very small electronic structures have reproducible properties they must be patterned reproducibly. Ideally one would like to achieve precise control on the molecular level on the structure of the electronically active device. The developing field of molecular electronics recognizes the importance of control on the atomic scale. This field of electronics capitalizes on the fact that molecules can be chemically prepared and subsequently they can be incorporated into electronic structures (also known as the bottom-up approach). In traditional molecular electronics, molecules are typically attached to metallic contacts in order to incorporate them into electronically functional structures. However, the metal to molecule contact typically has poor electronic characteristics. Furthermore, it is difficult to control the metal to molecule contact on the atomic scale.
Recent developments in nano-electronics have born out that graphite-based materials have many advantageous electronic properties over metals and semiconductors. These advantages include the very high current carrying abilities of graphitic structures, ballistic and coherent transport, and the fact that graphitic structures can be either metallic or semiconducting depending on their shape, as has been reported in the scientific literature.
Graphene structures transport electronic currents due to the π bonds. These π bonds result from the overlap of the pz electronic orbitals of a carbon atom with its three carbon neighbors. For extended graphite based structures, like graphite or graphite ribbons, the π bonds give rise to π bands. If the π bands intercept the Fermi level then these bands can transport electrical currents analogous to the electronic bands in metals. Hence, properly nano-patterned graphite ribbons can serve as wires to conduct electrical currents, that is, they can serve as electronic conduits. These principles form the basis of NPEG electronics.
In order to add electronic functionality to electronic structures, an electronic current must pass through a specific molecular structure whose electronic transport properties can be controllably modified. This is typically done by applying a gate potential that affects the electronic transport through the molecular structure. Alternatively, the molecular structure can be exposed to a chemical environment that affects the electronic transport through it. Hence, the molecular structure may be designed to be a chemical sensor. Magnetic fields may also affect the electronic transport through a structure.
Therefore, there is a need for an electronic device technology that includes graphitic structures that are functionalized.